Going to a private school in Norway — where public schools already have a very good English education program — I have learned the benefits of an early introduction to a language. From the first through third grades, most of our English education consisted of songs and fairy tales. This gave us a familiarity with the language that became crucial when we delved deeper in later years.
Our teachers in those early years had nowhere near perfect pronunciation. But their introduction to English still gave us a gateway to the language and culture, which otherwise would have proved difficult to penetrate.
Introduce English as a mandatory class from the fourth grade and the gross domestic product of Japan will increase as soon as these kids hit working age.